Yes, to open an existing pledge, click on a Due Amount from the constituent's Pledges tab or the main Pledges grid.

On the payment schedule, click on +Add Payments Due button. In the new dialog box, enter the number of payments in Add Number of Payments and then click OK. The payment schedule will be updated with the new payments and the total Pledge Amount will update at the top of the page to reflect the new total.

Click the green Save button at the top of the page to save the changes.